The present invention relates to a method of manufacturing piezoelectric devices for use as sensors, actuators and the like of various kinds and, in particular, to the piezoelectric devices using direct bonded quartz plate.
An example of the prior art method will be described with reference to FIG. 16 to FIG. 20.
A direct bonded quartz plate consists of two quartz plates 1 and 2, which are put together as shown in FIG. 16, and heated while being held together. The process of heating evaporates moisture present in bonding planes of both the plates, and facilitates bonding between these planes via oxygen atoms as intermediates.
The quartz plates bonded in this manner are provided with gold masks 3 and 4 on respective surfaces, and the quartz plates are dissolved simultaneously from both the surfaces by wet etching, as shown in FIG. 17. A process of cutting through the quartz plates is completed by the dissolution, as shown in FIG. 18, and this produces small chips of direct bonded quartz plate 5 as illustrated in FIG. 19. Following the above process, direct bonded quartz plate 5 is dissolved further by wet etching from both sides using upper and lower gold masks 3 and 4, to calibrate a resonance frequency as a resonator by way of adjusting a weight thereof, as shown in FIG. 20. This direct bonded quartz resonator is used for sensor and actuator as stated above.
A problem associated with the above example of the prior art has been that a sensitivity of the resonator constructed of the direct bonded quartz plate decreases in the wet etching process. The purpose of bonding the quartz plates 1 and 2, of which polarization axes are reversed, is to obtain twice as high sensitivity. However, the process of wet etching exposed sides of the quartz plates for calibration of the resonance frequency causes the quartz plates unsymmetrical in sectional shape of the dissolved surfaces as shown in FIG. 19, because of a difference in dissolvability attributable to crystal lattice of the quartz plates. When the dissolving process is continued further, the dissolution advances greatly toward an interior of lines Zxe2x80x94Z in the area of direct bond of the quartz plates 1 and 2, as shown in FIG. 20. Lines Zxe2x80x94Z in FIG. 20 are finite lines connecting between edges of upper and lower electrodes.
Dissolved portion 102 is an area where upper and lower quartz plates 1 and 2 are not bonded together. In other words, it reduces a bonded area of quartz plates 1 and 2 between the confronting upper and lower electrodes, and decreases the sensitivity as a consequence.
The present invention is intended to improve sensitivity of the direct bonded quartz piezoelectric device.
In order to achieve this object, the present invention discloses a method comprising the steps of directly bonding at least two quartz plates, dry etching the bonded quartz plates with plasma from one side of its surfaces, and dry etching the bonded quartz plates with plasma thereafter from the other side of the surfaces.
That is, the dry etching takes a task of physical cutting when the bonded quartz plates are dry-etched with plasma from one side and then from the other side of the surfaces as described above. Therefore, unlike the prior art method of using wet etching, this method hardly causes deformation due to the difference in etching rate of the quartz surfaces.
In other words, when the plasma dry etching is made on the front and back surfaces in this manner, the bonded quartz plates become such a shape that the perimeter protrudes outwardly around the bonded plane. Once the quartz plates are formed into the protuberant shape, there is produced only a shallow slit cut into the quartz plates at the protuberant ridge along the bonded plane, even after resonance frequency calibration is made by dissolving them with such a process as wet etching. If the cut-in slit is only so shallow as in this instance, a bonded area of the original quartz plates is not reduced, and thereby the invention provides the piezoelectric device with high sensitivity.